Upload
james-clements
View
216
Download
0
Embed Size (px)
Citation preview
7/28/2019 4 Allkanes Structure
1/22
Lecture 4
Alkanes
Structure and Nomenclature
7/28/2019 4 Allkanes Structure
2/22
Alkanes and Alkane Isomers
Alkanes: Compounds with C-C single bonds and C-H bonds only
(no functional groups) Connecting carbons can lead to small or large molecules
General formula CnH2n+2where the number of Cs is n
Alkanes are saturated with hydrogen (no more can be added They are also called aliphatic compounds
7/28/2019 4 Allkanes Structure
3/22
Name These Shapes
7/28/2019 4 Allkanes Structure
4/22
Name These Molecules
Decane
7/28/2019 4 Allkanes Structure
5/22
HYBRIDISATION OF ORBITALS
The electronic configuration of a
carbon atom is 1s22s22p2
1 1s
22s
2p
7/28/2019 4 Allkanes Structure
6/22
HYBRIDISATION OF ORBITALSThe electronic configuration of acarbon atom is 1s22s22p2
1 1s2 2s2p
If you provide a bit of energy you canpromote (lift) one of the s electrons
into a p orbital. The configuration isnow 1s22s12p3 1 1s2 2s2p
The process is favourable because the of arrangement of electrons;
four unpaired and with less repulsion is more stable
7/28/2019 4 Allkanes Structure
7/22
Alkane IsomersCH4 = methane, C2H6 = ethane, C3H8= propane
The molecular formula of an alkane with more than threecarbons can give rise to more than one structure
C4 (butane) = butane and isobutane ( 2- methyl
propane)C5 (pentane) = pentane, 2-methylbutane, and 2,2-
dimethylpropane
Alkanes with Cs connected to no more than 2 other Cs arestraight-chain or normal alkanesAlkanes with one or more Cs connected to 3 or 4 Cs are
branched-chain alkanes
7/28/2019 4 Allkanes Structure
8/22
Structural IsomersConstitutional isomers: Isomers that differ in the connectivity atoms
Branched vs straight chains
Cpds other than alkanes can be constitutional isomers of one anotherThey must have the same molecular formula to be isomers
7/28/2019 4 Allkanes Structure
9/22
Structures of Alkanes
Condensed formulas an alkane can be represented in a brief/ condensed form or inmany types of extended form
A condensed structure does not show bonds but lists atoms,such as
CH3CH2CH3 (propane)
CH3C(CH2)2CH3 (2,2-dimethylpropane)
7/28/2019 4 Allkanes Structure
10/22
Naming Alkanes
Compounds are given systematic names by a process that uses
Prefix-Parent-Suffix
Follows specific rules
Named as longest possible chain
Carbons in that chain are numbered in sequence
substituents are numbered at their point of attachment
Compound name is one word Complex substituents are named as compounds would be
7/28/2019 4 Allkanes Structure
11/22
Nomenclature of alkanes
No. of Carbons Formula Name (CnH2n+2)1 Methane CH4
2 Ethane C2H6
3 Propane C3H84 Butane C4H10
5 Pentane C5H12
6 Hexane C6H14
7 Heptane C7H168 Octane C8H18
9 Nonane C9H20
10 Decane C10H22
7/28/2019 4 Allkanes Structure
12/22
Alkyl Groups
Provides the substituent groups on the root name
Alkyl group obtained by removal of one H from an alkane (apart of a structure)
General abbreviation R (for Radical, an incomplete speciesor the rest of the molecule)
Name of the alkyl group is obtained by: replacing -aneending
of alkane with -ylendingCH3is methyl (from methane)
CH2CH3is ethyl from ethane
7/28/2019 4 Allkanes Structure
13/22
Types of Alkyl groups Classified by the connection site (carbon type)-
Primary, secondary, tertiary and quartenary a carbon at the end of a chain (primary alkyl group)
a carbon in the middle of a chain (secondary alkyl group)
a carbon with three carbons attached to it (tertiary alkyl group)
7/28/2019 4 Allkanes Structure
14/22
Properties of Alkanes
Called paraffins (low affinity compounds)because they do not react as most chemicals
They will burn in a flame, producing carbon
dioxide, water, and heat They react with Cl2 in the presence of light toreplace Hs with Cls (not controlled)
7/28/2019 4 Allkanes Structure
15/22
Physical Properties
Boiling points and melting points increase as size ofalkane increases
Forces between molecules (temporary dipoles,dispersion) are weak
7/28/2019 4 Allkanes Structure
16/22
Cycloalkanes
Cycloalkanes : alkanes that have carbon atoms thatform a ring (called alicyclic compounds)
Simple cycloalkanes rings ofCH2units, (CH2)n,or CnH2n
Structure is shown as a regular polygon with thenumber of vertices equal to the number of Cs (aprojection of the actual structure)
cyclopropanecyclohexanecyclopentane
cyclobutane
7/28/2019 4 Allkanes Structure
17/22
Complex Cycloalkanes
Naturally occurring materials contain cycloalkane structures
Examples: chrysanthemic acid (cyclopropane),prostaglandins (cyclopentane), steroids (cyclohexanes andcyclopentane)
7/28/2019 4 Allkanes Structure
18/22
Properties of Cycloalkanes
Melting points are affected by the shapes and theway that crystals pack so they do not changeuniformly
7/28/2019 4 Allkanes Structure
19/22
Naming Cycloalkanes
Count the number of carbon atoms in the ring and the number inthe largest substituent chain.
If the number of carbon atoms in the ring is equal to or greaterthan the number in the substituent, the compound is named asan alkyl-substituted cycloalkane
For an alkyl- or halo-substituted cycloalkane, start at a point ofattachment as C1 and number the substituents on the ring sothat the secondsubstituent has as low a number as possible.
Number the substituents and write the name
7/28/2019 4 Allkanes Structure
20/22
Cis-Trans Isomerism in
Cycloalkanes Rotation about C-C bonds in cycloalkanes is limited
by the ring structure
Rings have two faces and substituents are labeledas to their relative facial positions
There are two different 1,2-dimethyl-cyclopropaneisomers, one with the two methyls on the same side(cis) of the ring and one with the methyls on oppositesides (trans)
7/28/2019 4 Allkanes Structure
21/22
Stereoisomers Compounds with atoms connected in the same order but which
differ in three-dimensional orientation, are stereoisomers The terms cis and trans should be used to specify
stereoisomeric ring structures
Recall that constitutional isomers have atoms connected indifferent order
7/28/2019 4 Allkanes Structure
22/22
THE END